1. Field of the Invention
The present invention relates to a solid-state image pickup device used in video cameras or the like, and a method of manufacturing the same.
2. Description of the Prior Art
Solid-state image pickup devices are employed in video cameras and the like and have recently come to be used widely. As a solid-state image pickup device becomes higher in performance and in picture quality, the characteristics of the solid-state image pickup device become more demanding. In particular, when an intense light enters the solid-state image pickup device, white stripes appear on the monitor, which cause smear. This smear occurs when the light enters the vertical shift register area. It is therefore attempted to decrease the smear by disposing an aluminum light-shield film above the vertical shift register area.
A conventional solid-state image pickup device is described below while referring to the drawing.
FIG. 7 is a unit cell cross-section of a solid-state image pickup device in the prior art extending from the photo-electric converting part (hereinafter called photo diode) and vertical shift register part (vertical Charge Coupled Device (CCD) register part).
First, the p type well 2 is formed on the n.sup.- type silicon substrate 1. The n.sup.- type region 3 is a photo diode formed on the p type well 2. Also the p type region 4 is formed on the p type well 2. The p type region 4 prevents the electric charge which is generated in the n.sup.- silicon substrate 1 from diffusing into the vertical CCD register part, so generation of noise is prevented in the vertical CCD part. The vertical CCD register part is formed in the n type region 5. The p.sup.+ type region 6 is formed between the photo diode and the vertical CCD register part. When transferring the signal charge generated in the photo diode to the vertical CCD register part, the P.sup.+ type region 6 controls the potential distribution in the n.sup.- silicon substrate 1. In the solid-state image pickup device, the photo diode and vertical CCD register part make up a unit cell and unit cells are formed in a matrix. To electrically separate adjacent unit cells, the P.sup.+ type region 7 is formed. The P.sup.+ type region 7 is formed by ion implantation. To reduce the dark current the P.sup.++ type region 8 is formed on the surface of the photo diode. Thus, a buried type photo diode is formed.
Consequently, the gate oxide film 9 and silicon nitride film 10 are grown on the n.sup.- silicon substrate 1. A polysilicon film is deposited on the silicon nitride film 10. By etching this polysilicon film, the polysilicon electrode 11 is formed as a driving electrode.
By thermally oxidizing the polysilicon electrode 11, the polysilicon oxide film 12 is formed. Afterwards, the oxide film 13 is grown on the polysilicon oxide film 12. For the prevention of smear noise, the refractory metal light-shield film 14 is formed. By flattening, thereafter, the interlayer dielectric film 15, aluminum light-shield film 16 and protective film 17 are formed sequentially.
In the prior art, it is proposed in the Japanese Laid-open Patent Publication No. 1-253268 to form a refractory metal or a film containing a refractory metal in the region excluding the photo diode as the light-shield film, to form a reflow film on the refractory metal and heat it, and to connect wiring on the reflow film.
In this conventional method, when the refractory metal light-shield film 14 is disposed near the polysilicon electrode 11, the light shielding performance of the refractory metal light-shield film 14 is enhanced relative to light directly entering the polysilicon electrode 11 or light entering through the interlayer dielectric film 15 (beneath the aluminum light-shield film 16). Hence the smear noise is reduced.
The polysilicon oxide film 12 formed by oxidizing the polysilicon electrode 11 is reduced in its film thickness beneath the bottom edge of the polysilicon electrode 11. The thermal oxide film formed by thermally oxidizing the polysilicon is poor in insulation tolerance. The dielectric breakdown voltage of the thermal oxide film formed by thermally oxidizing the polysilicon is about 1/2 to 1/3 of that of the thermal oxide film formed on a single-crystalline silicon. To increase the dielectric breakdown voltage in the region beneath the bottom edge of the polysilicon electrode 11, it is required to sufficiently increase the film thickness of the polysilicon oxide film 12 or oxide film 13 formed by CVD.
However, when the interlayer dielectric film composed of the polysilicon oxide film 12 and oxide film 13 is increased in film thickness, smear is likely to occur. This is because the light reflected on the surface of the n.sup.- silicon substrate 1 on which the photo diode is formed enters the polysilicon electrode 11 through the interlayer dielectric film composed of polysilicon oxide film 12 and oxide film 13. As a result, the light-shielding effect of the light-shield film decreases, and the smear increases.
If the interlayer dielectric film is made thin to improve the smear, the dielectric breakdown voltage between the refractory metal light-shield film 14 and polysilicon electrode 11 drops. Due to the drop in dielectric breakdown voltage, the image pickup action may be disabled.
That is, the smear of the solid-state image pickup device is determined by the film thicknesses of the polysilicon oxide film 12 and oxide film 13 which are enough to maintain a sufficient dielectric breakdown voltage between the polysilicon electrode 11 and refractory metal light-shield film 14. Thus, in the prior art, it is difficult to decrease the smear sufficiently.
In addition, when the refractory metal light-shield film 14 is formed near the photo diode, the refractory metal atoms in the refractory metal light-shield film 14 diffuse into the oxide film and reach the silicon substrate, 1. In the photo diode, the diffused refractory metal atom may induce crystal defects. In the solid-state image pickup device, such crystal defects cause an image defect called white blemish on the solid-state image pickup device. The white blemish is a serious problem because it lowers the reliability of the solid-state image pickup device.
It is an object of the invention to solve such problems of the prior art by presenting a solid-state image pickup device free from smear, possessing a sufficiently high dielectric breakdown voltage, and capable of suppressing the occurrence of white blemish.